Geothermal water is a potentially important source of energy. Two major problems in utilizing geothermal water are corrosion and scaling. The major contributor to scale formation is amorphous silica. Silica exists as dissolved silicic acid or silicic acid salts in deep hydrothermal fluids at concentrations that correspond to the solubility of quartz at the appropriate temperatures and pHs. The concentrations of silica by weight in such fluids is less than 0.2%. As the fluid rises to the surface and is cooled and/or flashed, the silica concentrations may exceed, many times over, the solubility of the amorphous form and precipitation occurs via the formation of polymeric silica structures. A number of installations have suffered operating problems that follow precipitation of the silica in well liners and equipment. The mechanism of polymerization and its relationship to scale buildup are not well understood. Many methods have been proposed over the years for removal of the silica from such waters. Typical types of processes are the addition of quicklime, which precipitates silica as calcium silicates, use of electrical potentials and dilution with fresh water.
Reinjection at liquid dominated geothermal fields is the obvious method to reduce drastically environmental impact. In addition, it may reduce ground subsidence rates and could decrease the rate of depletion of the natural resource by replacing unused mass and enthalpy. On the other hand, the useful reservoir lifetime might be foreshortened if reinjected silica were to precipitate in the aquifer and reduce the latter's permeability. However, gaps in the knowledge of silica chemistry have militated against routine reinjection. Despite more than three decades of investigation, neither the mechanism not the kinetics of silica polymerization have yet been characterized satisfactorily.
In my article which appeared in The AIChE Journal volume 22, No. 5, September 1976, page 817, I described a novel approach to the elimination of silica scaling in a heat exchanger tested by Kunze et al. In this test a synthetic geothermal fluid transferred heat to a second working fluid using a fluidized bed of sand particles as the heat exchange medium. The abrasive action of the sand effectively eliminated scale buildup on the heat transfer surfaces, and high heat transfer coefficients were maintained. In a paper given by me at the Proceedings of the Second United Nations Symposium on the Development and Use of Geothermal Resources, San Francisco, CA. on May 20-29, 1975, Volume 2, page 1323-1327 (1976) I suggested the possibility of passing supersaturated solutions through a sand filled, fluidized bed, heat exchanger in which the presence of microscopic nucleation centers might accelerate the precipitation of the silica from the solution.
Liquid fluidized beds have not been utilized to removed a dissolved species. On the other hand such systems have been utilized to remove suspended solid particles, some of which may result from a chemical reaction. Such a system is the removal of carbonic acid species from sea water by the addition of slaked lime as shown in U.S. Pat. No. 3,075,828 issued on Jan. 29, 1963 to Tsuneo Kato, Iwao Kanuo and Kenji Takeo to create a suspension which is passed through a fluidized bed.